the main factors for the migration and transformation of heavy metals in soil include the lixiviation of rocks by water, precipitation and adsorption . because the lixiviation of feldspar and the calcium carbonate which are abundant in loess increase the concentrations of alkaline ion HCO3- , alkaline metals, and alkaline earth metal ions including na and ca2 , ph of soil is increased. meanwhile, ph is an important influential factor for the adsorption and desorption, migration and available forms of heavy metals 24,25 . although the addition of acid sludge (Ph=6.38) can reduce the alkalinity, of loess, the experimental soil was still in alkalinity for its ph =7.79 , which had little influence on the activity, migration and toxicity of heavy metals. the components determining the alkalinity included HCO 3 , OH and CO ions, which can generate the hydroxide and carbonate precipitation by reacting with heavy metal ions, enabling Cd and Ni to produce stable precipitates in soil. furthermore, when composted sludge containing abundant organic mattere is applied in loess, it can reduce the absorption of Cd by plants by improving the content of organic matters in soil 26 . according to existing studies, multivalent cation is capable of linking the humic molecules together to form a chain structure, leading to the coagulation 27,28 the two effects mentioned above are favorable to the stabilization of the heavy metal fraction.